Considering the case of a surface with a cavity recessed in the surface, when the surface, and hence the recessed cavity, is moving in ambient fluid, for example when an aircraft with an open bay, for example an open weapons bay or an open undercarriage bay, is moving in air, a shear layer is formed between (i) the moving ambient air that is flowing across the surface and across the top of the recessed cavity, and (ii) the static air in the cavity (from the reference point of the aircraft). A vortex is shed from the cavity leading edge and grows as it travels down the shear layer and impacts on the aft (trailing) wall of the bay resulting in the emission of noise. Also the acoustic wave travels back upstream inside the bay. The fluctuating pressure of the acoustic wave may either result in vortices being shed from the leading edge cavity lip or an increase in the growth rate of the vortices such that a series of vortices is formed down the shear layer at a preferential rate which is related to the frequency of the upstream acoustic wave. The vortices grow into large scale structures as they propagate downstream in the shear layer and then impact the aft (trailing) wall of the bay at a characteristic rate. This results in acoustic noise being generated at a characteristic rate which may be described as acoustic tones of a characteristic frequency.
The frequency of the tones may be formulated using Rossiter's equation. It can be seen that here is a feedback loop formed by the passage of the vortices and the upstream propagating acoustic wave.
It is known in aircraft to employ spoiler arrangements to divert the airflow over and beyond a recessed cavity, i.e. over and beyond any boundary layer or prospective shear layer so that the above described effects do not take place. It is further known to include holes in such spoilers for the purpose of reducing the weight of the spoiler. Accordingly, the holes are sized and/or spaced so as to provide weight loss without substantially disrupting the operation of the spoiler in diverting substantially all of the impinging fluid flow. In other words, it is the case that such prior art arrangements reduce or remove the size of the shear layer. It is understood that accordingly for such known spoilers the proportion of the active spoiler area that is taken up by the holes is usually less than 30%. A report (Technical Report AFFDL-TR-79-3003 published February 1979 by Air Force Flight Dynamics Laboratory, Air Force Wright Aeronautical Laboratories, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio 45433, USA), titled “Evaluation of F-111 Weapon Bay Aero-Acoustic and Weapon Separation Improvement Techniques”, authored by Rodney L. Clark, appears to mention, but not evaluate, a spoiler with the proportion of the area taken up by the holes being 50%.
It is also known to employ meshes or plates with holes over the entrance of air intakes or the entrance of other tunnel like arrangements, for the purpose of preventing physical objects to enter the air intake or other tunnel-like arrangement. Examples of publications in which such meshes and/or plates are: DE 10 2005 007940 A1, U.S. Pat. No. 2,663,993 A, U.S. 2009/045286, and GB 614 274 A. It is noted that such arrangements, even if considered as including a cavity as such, do not include cavities that are of the type being addressed in the present invention which on the contrary are cavities recessed in a surface.
U.S. Pat. No. 5,699,981 discloses an aircraft cavity acoustic resonance suppression system which comprises a small diameter, substantially cylindrically shaped member disposed substantially parallel to and spaced up to a distance corresponding to about three airflow boundary layer thicknesses from the surface of an aircraft near the leading edge of the cavity and transverse to airflow thereacross. An actuator is provided to select the adjustment of the spacing between the member and the aircraft surface according to different operational speeds and hence different operational boundary layer thicknesses as the member's spacing of about three airflow boundary layer thicknesses from the surface of the aircraft is disclosed as critical.